Barkeep, as Jay and Randy have pointed out, if it hums even when no other connection other than the power cord exists, it can't be a ground loop hum because there's no connection to form a loop. That points clearly to the internal sub amp itself and since it isn't a mechanical hum from vibrations in the transformer, but an electronic hum coming from the driver, there appears to be a fault in the power supply section. The rectifier and filter capacitors do the job of converting AC from the wall outlet into clean DC to be used by the amplification section. It's likely that the rectifier or a capacitor is going bad and needs to be replaced if the hum is too annoying.
Hmm, that's a new and interesting perspective. Thanks. Now if I only knew which component(s) to change out. There are two big caps at 6800 uF, and some other components. Here's a pic. Is there any way to test the components to see which one(s) need to be replaced?
